fix: improve pill contrast by using WCAG contrast ratios on composited backgrounds

Pills with semi-transparent backgrounds were using raw color luminance to pick
text color, ignoring the page background showing through. This caused unreadable
text on dark themes for mid-luminance colors like orange. Now composites the
rgba background over the actual page bg (dark/light) before computing WCAG
contrast ratios, and centralizes the logic in a shared color-contrast utility.

Co-Authored-By: Paperclip <noreply@paperclip.ing>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/pages/IssueDetail.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useRef, useState, type ChangeEvent, type DragEvent } from "react";
+import { pickTextColorForPillBg } from "@/lib/color-contrast";
 import { Link, useLocation, useNavigate, useParams } from "@/lib/router";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { issuesApi } from "../api/issues";
@@ -767,7 +768,7 @@ export function IssueDetail() {
                   className="inline-flex items-center rounded-full border px-2 py-0.5 text-[10px] font-medium"
                   style={{
                     borderColor: label.color,
-                    color: label.color,
+                    color: pickTextColorForPillBg(label.color, 0.12),
                     backgroundColor: `${label.color}1f`,
                   }}
                 >